Pipe pressure help

  1. Did an experiment in a school's heat system. Turned off the unit ventilator valves all at once. The pipe pressure raised significantly causing the circulation pumps to back off. Some notes, this is a reverse return loop, hot water flowing (not steam), unit ventilators are 2-pipe.

    Question: why did pressure increase when uni-vents were throttled? I thought Bernoulli said that narrowing pipes would cause velocity to increase and pressure to decrease...shutting off all branches should narrow all flow to the supply line only, right?
     
  2. jcsd
  3. Yeah, at the pipe sesction you are looking at, sure. Upstream at the pump end, the pump has to work harder to achieve the same amount of flow. In your case, if you shut all valves, there is no flow and the pump just spins the water in its casing around. Is that good for your pump? No bypass in your system?

    what does that actually mean.
     
  4. russ_watters

    Staff: Mentor

    Probably that they are on variable speed drives and slowed down.
     
  5. russ_watters

    Staff: Mentor

    What pressure, where? There's velocity pressure, static pressure, total pressure, differential pressure....

    Bernoulli's principle is talking specifically about static pressure inside the venturi vs right next to it. So unless you are measuring the static pressure inside the valve itself, you won't see a drop. In your system, the control point is almost certainly the static pressure in the main, who's size you didn't change.

    Bernoulli's principle is also talking about steady flow along a streamline: pressure difference between two parts of the same system at the same time. You measured one point on the system at different times.
     
  6. Couple of answers. Water in the supply loop either goes through the uni-vent or continues to the end, where the return loop collects all uni-vent discharge and the supply loop remnants. Thus, the supply loop simply dumps into the return loop if the uni-vent valves all shut, which is the same as a bypass I guess. The circulation pumps are VFDs, and they spin faster when more pressure is needed and less when the pressure is higher than needed. Throttling all of the valves made the pressure increase, so the VFDs slowed down. I can't explain why, and am looking for a mathematical relation to help understand this.
     
  7. Mentor, you are asking the right questions. This shows my ignorance in heating systems. The pressure differential is what tells the VFDs to slow down or speed up, and that is the pressure I am trying to understand. That pressure increased causing the VFDs to slow down. Do you know why?
     
  8. russ_watters

    Staff: Mentor

    Pumps have performance curves. When you introduce a restriction against their flow, their performance follows the curve up and back, to a higher pressure and lower flow state. A similar thing happens when you put your thumb on the outlet of a garden hose. Slowing the flow allows the pressure to build.
     
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